package com.walmartlabs.concord.common;

/*-
 * *****
 * Concord
 * -----
 * Copyright (C) 2017 - 2020 Walmart Inc.
 * -----
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * 
 *      http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 * =====
 */

import java.util.function.Supplier;

/**
 * A memoizing {@link Supplier}. Not thread-safe.
 */
public class MemoSupplier<T> implements Supplier<T> {

    public static <T> Supplier<T> memo(Supplier<T> delegate) {
        return new MemoSupplier<>(delegate);
    }

    private final Supplier<T> delegate;

    private volatile boolean initialized;

    T value;

    public MemoSupplier(Supplier<T> delegate) {
        this.delegate = delegate;
    }

    @Override
    public synchronized T get() {
        if (!initialized) {
            T t = delegate.get();
            value = t;
            initialized = true;
            return t;
        }
        return value;
    }
}
